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Since the energy crisis of the 70's, many folks have been turning down their thermostats in the evenings to save energy. Automatic thermostats typically offer a ten-degree or more temperature variation in their programming.
But if the temperature outside is very bitter… in the teens or lower… a 10-degree setback can be disasterous for some homes! The reason is simple. Lowering the thermostat puts your heating system into a long "dormant" cycle. Your furnace will not start again until the temperature in the house has dropped those ten degrees. In a well insulated home this could take many hours during which the temperature in the outside walls and in less-than-adequately insulated portions of your home can drop below freezing. This can cause drinking water and heating pipes in these areas to freeze.
If you really want to play it safe:
1) Never lower your thermostat more than 5 degrees in a four-hour period if the outside temperature drops below 20 degrees.
2) During these very bitter periods, keep the inside temperature above 60 degrees.
2) If your house has a history of frozen pipes you should not lower the temperature at all until you have protected the pipes in those problem areas with extra insulation or even automatic electric heating tapes.